This invention relates to dielectric materials for use in electrical applications, and more particularly to glass ceramic materials having controllable temperature coefficients of dielectric constant or capacitance.
Many uses for dielectric materials require a temperature coefficient of capacitance (TCC) value which is near zero for the temperatures involved, typically -55.degree. to 125.degree. C. for military specifications. That is, the rate of change in capacitance with respect to temperature must be near zero throughout the temperature range. Since capacitance is directly related to the dielectric constant of a material, the temperature coefficient of the dielectric constant must also be near zero. Such uses for these dielectric materials include capacitors (so-called NPO capacitors), resonators in microwave filter networks, and as microwave substrates below 5GHz. Military communications systems such as long range radar using microwaves and millimeter waves also require the use of low dielectric loss, temperature stable dielectric materials. Still other uses of such dielectric materials require specific TCC's (i.e., 100 ppm/.degree.C.) for the temperatures involved.
Previously, dielectric materials meeting the above requirements were achieved by mixing appropriate quantities of ceramic materials which have both positive and negative TCC's. For example, mixtures of zirconates having a positive TCC and zirconates or titanates having a negative TCC have been used. See Kell, 38 J. Science and Technology No. 1 (1971). More recently, Kolar et al., 27 Ferroelectrics 269 (1980) have utilized ceramic compositions containing barium titanate (BaTiO.sub.3) and neodymium oxide (Nd.sub.2 O.sub.3).
However, such mixtures of ceramic materials have several major drawbacks to their use. Stability of the compositions is often compromised due to combinations of domain-wall effects and intergranular or interfacial phenomena. Domain, or so-called Bloch, walls consist of transition layers of a thickness of a few hundred lattice constants between adjacent ferromagnetic domains which adversely affect the properties of the compositions.
Moreover, ceramic materials require densification to avoid porosity and its associated moisture problems. Even the most sophisticated sintering or hot-pressing techniques may not be adequate to produce a theoretically 100% dense ceramic body. Finally, ceramics can be formed into the necessary geometric shapes only by expensive grinding, lapping, and polishing procedures. Molding techniques can be used only for the simplest of geometries.
Accordingly, the need exists for materials having the requisite TCC values which exhibit low dielectric losses at typical temperatures of operation, are pore free, and can be formed readily into complex geometrical shapes.